The contemporary design of spark ignited internal combustion engines must cope with the increasingly stringent regulations on pollutant emission. Accordingly, automotive engineers strive for designing engines with low fuel consumption and low emission of pollutants, which implies including electronic devices capable of monitoring the combustion performance and emissions in the exhaust gases.
The issue of fuel economy has been addressed i.a. by varying the injection schemes. Currently, direct injection engines and in particular gasoline stratified charge engines are considered to be very efficient in terms of fuel economy.
One requirement to reduce emissions from a spark ignited internal combustion engine is an accurate control of the combustion air/fuel ratio. This is usually done by metering a precisely controlled amount of fuel based on a measured or inferred air charge mass inducted into the engine; many control schemes are known in the art to control the air/fuel ratio. It is e.g. customary to install an oxygen sensor in the engine exhaust line pipe and to use the sensor output as a feedback signal for closed loop fuel control.
U.S. Pat. No. 6,382,198 describes a direct injection engine with an enhanced fuel control using a single oxygen sensor as combustion performance indicator. The Engine Control Module (ECM) is capable of determining the actual air/fuel ratio corresponding to each individual cylinder from the combined flow of exhaust gases; this function is known as ICFC (Individual Cylinder Fuel Control). Conventionally, the ECM develops a fuel command pulse width for each of the injectors that corresponds to the driver's requested torque. For this purpose, a lookup table is used that stores fuel amounts in function of e.g. engine speed, manifold air pressure, and other parameters. The ECM also uses a table storing closed-loop fuelling corrections, which is known as block learning memory (BLM). As it is known in the art, the BLM table entries are determined based on the oxygen sensor response, which when adequately filtered, provides a measure of the deviation of the average engine air/fuel ratio from stoichiometry (average here means for a bank, i.e. a set of cylinders connected to the same exhaust manifold). The values from the base table and BLM are used to determine a global fuel amount. Additionally, an ICFC module determines, also based on the oxygen sensor response, a cylinder specific fuel error that is used to develop individual cylinder correction factors applied to the global fuel. This final fuel amount is then converted into a pulse width command, which typically involves a lookup table storing fuel amounts vs. pulse widths.
This control strategy is already quite sophisticated and does indeed allow an enhanced control of fuel injection. A problem that however has recently arisen in injection control is that advanced, complex fuel injectors, in particular those used for stratified charge engines, do not have easily predictable flow performances, which results into significant performance deviation or variability between injectors of a same design.
A further parameter affecting the injected fuel quantity is the response time of the injector. Indeed, conventionally with electromagnetic actuators a certain time period elapses between application of the command signal and the moment the actuator actually starts moving; or between the moment the command signal ends and the injector actually reaches its closed position. The knowledge of the response times (or response delays) at switch-on and switch-off thus allows for a more precise control of the actuator. WO 03/023211 e.g. describes a method of determining response times of electromagnetic devices. The determination of injector response times at switch on and switch off based on current detection is described; the determination of the response time at closing is also described based on voltage detection.
Deviation and variability between injectors are usually due to the dispersion of the injectors' characteristics linked to the production process spread and/or to the time-drift variations of the same characteristics due to ageing. Thus, fuel injector flow variations need to be corrected.
The problem of fuel variability is particularly critical for low fuel injections, i.e. when injecting small or minute fuel amounts.